Could she have spent her last few minutes with the boys before shooting them, but I suppose this means Nevill just let her wonder off in a rage with the rifle upstairs?
Nevill would never let her wander off, armed or not, yet we have the fact that, if we accept Jeremy's defence, Nevill has to make a phone call to Jeremy at some point. This can only mean that Sheila was with Nevill or close by when Nevill made the call. There is also the uncontested fact that the boys were asleep when shot. I think the facts default to her not spending time with them.
I believe my scenario stands. I do not say Jeremy is innocent, but in broad terms, it is the only plausible alternate scenario if Jeremy is innocent.
In response to Adam's poll, I say Sheila was standing in the kitchen, opposite Nevill, with the rifle, as Nevill made his call to Jeremy. When Jeremy answers, she runs out of the kitchen and Nevill follows her, terminating his call with Jeremy.